config.mk: Delete unnecessary code

Currently no makefiles (board-specific config.mk)
set the following variables:

CONFIG_SPL_TEXT_BASE
CONFIG_UBOOT_PAD_TO
CONFIG_RESET_VECTOR_ADDRESS
CONFIG_TPL_PAD_TO

For all target boards using above macros
they are set in header files (include/configs/*.h),
so we do not need to set them as CPPFLAGS.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>